1. Field of the Invention
This invention relates generally to anti-skid braking systems for wheeled vehicles. More particularly, this invention relates to an improved system and method for controlling wheel slip in a multiple wheel braking system. This improved system is particularly useful on vehicles with a wide wheel base such as general aviation aircraft. This improved system optimizes the braking performance of such paired multi-wheeled vehicles while minimizing directional deviation.
2. General Background and State of the Art
Anti-skid and automatic braking systems commonly have been provided on multi-wheeled vehicles such as general aviation and commercial aircraft to aid the deceleration of the vehicle. Modern anti-skid systems typically optimize braking efficiency by adapting to runway conditions and other factors affecting braking to maximize deceleration, corresponding to the level of brake pressure selected by the pilot. In conventional antiskid systems, brakes are typically applied mechanically via a metering valve by the pilot, and as soon as the wheel brake pressure approaches the skid level, such as when an initial skid is detected, a brake pressure value is used to initialize the antiskid control system.
In aircraft applications, rapid pedal application by an aircraft pilot during landing can often create deep initial skids before an effective anti-skidding brake pressure or brake torque is determined and skidding is effectively controlled by conventional antiskid and brake control systems. Reducing initial skids and maximizing braking efficiency would result in shorter aircraft stopping distances, which allow the aircraft to land on shorter runways, and can result in reduced tire wear.
Conventional skid control systems typically include slip indicators having a wheel speed transducer and a brake pressure sensor for each wheel brake of the vehicle. The wheel speed transducers measure wheel speed and generate wheel speed signals that are a function of the rotational speed of the wheel. The wheel speed signals are typically converted to a signal representing the velocity of the wheels, and compared with a reference velocity of the aircraft. This comparison may generate a wheel velocity error signal indicative of the difference between the wheel velocity signals from each braked wheel and the reference velocity signal. The output of the velocity comparator is referred to as “velocity error”. The velocity error signals typically are adjusted by a pressure bias modulator integrator, a proportional control unit, and a compensation network. The output of such logic circuits are summed to provide an anti-skid control signal which is received by a command processor. The pressure bias modulator integrator dictates the maximum allowable control pressure level during braking. That is, when no skid (or slip) is detected, this integrator allows full system pressure to the brakes and allows less pressure as the skid is detected.
Conventional and recently invented anti-skid control systems are well-known to those of ordinary skill in the art. Some recently invented anti-skid control systems are described in U.S. Pat. Nos. 4,562,542; 6,655,755; and 6,659,400. Those of ordinary skill in the art of anti-skid brake control are familiar with the advantages and disadvantages of the different control systems described in these and other patents.
Conventional anti-skid systems have processed the pressure bias signal in at least two different ways: either as paired or individual control signals. In paired skid control systems the skid control signal from a single wheel is sent to both wheel's pressure control valve. In individual skid control systems, each wheel generates an individual skid control signal which is sent to its own pressure control valve.
Paired skid control systems typically take either the signal from the first wheel to indicate a slip or from the wheel indicating the greatest slip and uses that signal to modulate the brake pressure to both or all wheels. Such systems prevent directional deviation from being generated by the anti-skid system. In vehicles with wide wheel bases, such as generate aviation aircraft, small variations in the brake pressure between the spaced apart wheels may induce directional deviation. One drawback of these systems is reduced braking efficiency. Since the brake pressure to all wheels is being reduced in response to the wheel experiencing the lowest friction coefficient, available brake pressure to the other wheels is being sacrificed.
Individual skid control systems take the signals from each individual wheel and responds with a signal to modulate brake pressure sent to each individual wheel's pressure control valve. That is, the control system modulates the brake pressure to each individual wheel according to the slip conditions experienced by each wheel. These systems maximize the braking efficiency of the entire vehicle by allowing the maximum allowed braking pressured for the conditions experienced by each wheel. One drawback of these systems is directional deviation caused by the anti-braking system. As greater brake pressure is applied to a wheel experiencing less slip, torque is generated by the different forces applied to different wheels. Torque on the vehicle is experienced as directional deviation.
A need therefore exists for an anti-skid braking system for a multi-wheeled vehicle which optimizes the braking efficiency of the vehicle while minimizing directional deviation. The present system meets these and other needs.